Clean public boundary vocabulary

[chubes4]

Summary

• Reframes public docs around WP Codebox as the portable integration surface.
• Replaces named backend/internal vocabulary with generic host job system, agent execution substrate, workspace backend, runtime backend, and contained WordPress runtime terms.
• Tightens boundary-language tests so public docs ban implementation vocabulary and no longer require generic Data Machine inputs.

Tests

• npx tsx tests/docs-boundary-language.test.ts
• npm run test:public-api-contract
• npm run test:production-boundary-enforcement

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Drafting documentation/test changes, running verification, and preparing this PR.